Speed reduction of ele motor

[T.SwinfieldParticipant @t-swinfield]

Hi, what is the best way to reduce the speed of a 240v  4 pole electric motor.

Hoping you can help.

Thanks Tony

[Peter Cook 6Participant @petercook6]

Permanently – swap the pulleys/gear train

Variably – install a VFD

[noel shelleyParticipant @noelshelley55608]

You do not say what type of drive ? You don’t say what speed you need ? Beltdrive is easy, use different size pulleys ! Chain the same ! A speed changing gearbox ! A 4 pole will be about 1430RPM, Single phase will not be happy on simple VFD. Change motor to required speed or go to 240v 3phase motor and VFD

Give us more clues ! Noel

[Mark RandParticipant @markrand96270]

To add to the above, do you need the same torque at a lower speed or a greater torque? If you only need the same torque and it’s a three phase motor, then a VFD will do the job. If you need greater torque (lathes, bandsaws, milling machines etc. generally want greater torque at lower speeds due to larger work/cutter diameters or tougher stock), then pulleys, sprockets or gears are the solution.

[Howard LewisParticipant @howardlewis46836]

If the motor is running on AC, the speed will be related to the frequency of the suppy, and cannot be altered, unless the power supply is through a VFD.

A brushed DC motor speed can be controlled by varying the supply voltage, but at the cost of reduced powere as the voltage is reduced.

Brushless DC motor speeds can be controlled electronically.

If the existing motor is AC; Sounds like it , being 4 pole. On 50 Hz mains, I would expect a shaft speed circa 1470 rpm.

If the objective is to vary the speed of the machine driven by the motor, this can be achieved in a variety of ways, but only one will even approach being infinitely variable.

This is a variator, usually in the form of two, opposed conical belt pulleys, one of which can be used to move the belt up and down the cone, thus varying the ratio.

The other means provide speed changes for the output shaft, in fixed steps. Gears, Chain and sprockets, or belts and pulleys. Gears and chains provide a positive drive, which unless cogged belts and pulleys are used, V or flat belts do not.

I hope that this clarifies your thoughts

Howard.

 

[Nigel Graham 2Participant @nigelgraham2]

Plain Vee-belts and pulleys carefully selected for the task, and tensioned correctly, are fine. For larger powers use double or triple belt drives.

They have the advantage over toothed belts, chains or gears that if something jams they have some chance of slipping enough to give you time to cut the power.

[SillyOldDufferModerator @sillyoldduffer]

[noel shelleyParticipant @noelshelley55608]

The original question stating only 240v 4pole leads me to think it is single phase not dual voltage in delta ? Almost all options have been offered, all we need now is for matters to be clarified so we can know which options are valid !  Noel.

[File HandleParticipant @filehandle]

“Not the law exactly, but belts and chains usually require the machine to be stopped for speed changes. Doesn’t matter on most machine tools, but no good for vehicles.”
Except that some vehicles do use a variable belt drive. It means that you can vary the speed without changing the engine speed, thus keeping maximum power.

[T.SwinfieldParticipant @t-swinfield]

Thanks to you all for spending the time to reply to my query, I have made a power hacksaw ,with the intention of using a spare motor that I have ,I made a gearbox using myford change gears which by calculations would have given me a speed of 100 rpm. The motor would be connected to the gearbox by V belt, unfortunately the electric motor does not work so I need another motor, I do realise a 3 phase motor with inverter would be the best option, but would be expensive, that’s when I thought of a single phase motor with speed control would be better than the gearbox. I’m still not sure which way to go .

Thanks again to you all.

Regards Tony

[Roderick JenkinsParticipant @roderickjenkins93242]

Any reduction of speed from the  motor’s initial design, even if possible, is probably not desirable- especially a big reduction which will compromise the torque available.

I used to have a home made power hacksaw that was given to me. This used a timing type belt and pulleys that gave a bigger reduction ratio than would be possible with V belts.

Rod

[noel shelleyParticipant @noelshelley55608]

Tony, Rod is touching on my thoughts, ie the effect of running a motor at less than its rated speed ! I suspect that you will need reasonable torque on the hacksaw to avoid circumstances where it will stall the motor and spoil it ! Simply running a motor slowly, even a 3phase one and doing away with the gearbox is not an option ! You will need both a gear box and belt drive OR a 2stage belt system to get the sort of reduction you indicate for 100RPM Variable speed is one thing, what you need is speed reduction by mechanical means to give the torque. My old rapidor hacksaw had 2 stage belts, modern units use worm drive boxes and a 3 step pulley. Good luck. Noel.

[Howard LewisParticipant @howardlewis46836]

As Rod and Noel say, running a motor below its normal speed can result in problems.

The slower speed will mean that less air is forced through the motor by the fan, so it could overheat.

Also, slow running is likely to result in the windings  generating a lower back EMF, so that the current draw increases, and with it the heat input.

This would be a double whammy, making damage even more likely.

IMO, keep the motor at designed speed, and use belt and pulleys, or chain and sprockets to reduce the speed of the hacksaw.

Howard

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